Since information societies are now a reality, a large number of displays, such as liquid crystal displays (LCDs), plasma display panels (PDPs), electrophoretic displays (EPDs), and the like, are being developed or commercialized, and indoor displays are continuously being increasing in size and decreased in thickness, while portable outdoor displays are being miniaturized and lightened. In order to improve functionality of the displays, various kinds of optical films have been used.
Materials used for the optical films generally need to have properties such as high degrees of light transmittancy, optical isotropy, non-defect containing surfaces, high heat and moisture resistance properties, high degrees of ductility, high degrees of surface hardness, low rates of shrinkage, ease of processability and the like.
In a polarizing plate, as a protective film for protecting a polarizer formed of a polyvinyl alcohol material, a triacetyl cellulose (TAC) film having properties such as high degrees of light transmittancy, optical isotropy, non-defect containing surfaces, and the like, has been generally used on one surface or both surfaces of the polarizer. However, since triacetyl cellulose (TAC) films are vulnerable to heat and moisture, in the case of the long-term use of such films under conditions of high temperature and moisture, defects such as a light leakage phenomenon in which an excessive amount of light is leaked from edges of the film due to a lowering in a degree of polarization and moisture degradation, or the like, may be generated, to degrade durability.
Thus, protective films formed of various materials, capable of replacing the triacetyl cellulose (TAC) film have been being developed, and for example, a method of using polyethylene terephthalate (PET), cycloolefin polymer (COP), an acrylic film, and the like, alone or in combination, has been suggested. In particular, the acrylic film is known to have advantages in terms of price, as well as having optical properties and durability.
Meanwhile, as an adhesive used to attach a protective film to a polarizer, a aqueous or non-aqueous adhesive is commonly used. However, since the acrylic film has a high degree of surface friction force, a sufficient degree of adhesion between the polarizer and the film may not be ensured, even in the case of using the adhesive described above.
In addition, due to high surface friction force of the acrylic film, a blocking phenomenon in which film surfaces are in contact with and adhered to each other may be generated during or after the winding of the acrylic film. In order to solve the defect, although a method of filling a film with a small amount of rubber particles or inorganic particles at the time of forming the film has been suggested, such a film is not suitable for being used as an optical film, due to disadvantages thereof such as a lowering in transparency of the film in accordance with an increase in haze, a degradation in stretchability thereof and the like.
Meanwhile, if necessary, an optical film generally includes a surface coating layer in order to complement deficient properties due to inherent characteristics of a material thereof and to realize additionally required display functions. In particular, in a case in which the optical film is disposed in an outermost portion of a display, it may be significantly important to impart functionality such as anti-glare, anti-reflection or hard coating properties to the surface coating layer.
However, when such a surface coating layer is formed on a surface of the polarizing plate, an acrylic film may not be coated on the surface due to insufficient solvent resistance thereof and may be disadvantageously melted.
Therefore, the development of a protective film, enabling a surface coating layer thereof to be easily formed and having slip properties suitable for a roll-to-roll process performed during the production of a polarizing plate, as well as having excellent durability and transparency and high levels of adhesion with respect to a polarizer, has been demanded.